1. Field of the Invention
This invention relates to a magnetic tape drive of a helical scan system for storing digital data.
2. Description of the Related Art
Demands for large capacity magnetic tape units for computers for storing data and high speed data transfer grow as the information processing amount increases year by year.
The systems of recording digital data on magnetic tape include a fixed head system which forms recording tracks in the longitudinal direction on magnetic tape by means of fixed heads and a helical scan system which forms recording tracks slantwise with respect to the longitudinal direction of the magnetic tape by means of rotary heads. At present, the fixed head system is adopted mainly with magnetic tape drives for large scale computers, but the helical scan system is useful for high density recording and high speed data transfer, and is expected to be the next leading recording system. In a field using comparatively small computer systems such as personal computers and workstations, magnetic tape drives of the helical scan system using DAT tape for digital audio and 8-mm tape for VTR are already put into products. Information thereon is provided, for example, in "Television GAKKAISHI" Vol.43, No.5 (1989) pp.483-489.
By the way, one of the magnetic tape drive functions is a re-try function. If data read from one track of magnetic tape contains an error, the re-try function is to repeat a read of the data on the same track until the correct data are obtained. The causes of an error occurring in read data include a data dropout caused by a read head clogged by adhesion of magnetic powder of tape or other dirt thereto. Since the error may occur temporarily, the correct data can be obtained by a reread. Thus, the re-try function is an important function generally executed at recording and reproducing apparatus handling digital data.
The re-try operation of a magnetic tape drive is performed as follows:
When an error is detected in read data, the magnetic tape is run in the opposite direction to the tape run direction at reproducing (forward) and is rewound to a preceding position from the error occurrence position by the approach distance required to raise the tape speed to the normal speed then is started at that position in the forward direction for again reading the data.
However, when such a re-try operation is executed, it takes time until the re-read operation of magnetic tape is started in such a manner. As a result, the throughput of the system is lowered. Shortening the re-try operation time is an important subject.
Particularly, the structure of a tape path is complicated in the helical scan system compared with the fixed head system, thus it is difficult to change the tape run operation at high speed, and the re-try operation time is furthermore prolonged.